ΕΛΛΗΝΙΚΗ ΕΠΙΤΡΟΠΗ HELLENIC CHAPTER ΥΔΡΟΓΕΩΛΟΓΙΑΣ OF IAH ΣΥΝΔΕΣΜΟΣ ΓΕΩΛΟΓΩΝ & CYPRUS ASSOCIATION OF GEOLOGISTS & ΜΕΤΑΛΛΕΙΟΛΟΓΩΝ ΚΥΠΡΟΥ MINING ENGINEERS 11th INTERNATIONAL HYDROGEOLOGICAL CONGRESS 11ο ΔΙΕΘΝΕΣ ΥΔΡΟΓΕΩΛΟΓΙΚΟ ΣΥΝΕΔΡΙΟ CONFERENCE PROCEEDINGS VOLUME ΠΡΑΚΤΙΚΑ ΣΥΝΕΔΡΙΟΥ ΤΟΜΟΣ 2 ORGANISED BY/ ΔΙΟΡΓΑΝΩΣΗ: HELLENIC CHAPTER OF INTERNATIONAL ASSOCIATION OF HYDROGEOLOGISTS- IAH ΕΛΛΗΝΙΚΗ ΕΠΙΤΡΟΠΗ ΥΔΡΟΓΕΩΛΟΓΙΑΣ CYPRUS ASSOCIATION OF GEOLOGISTS AND MINING ENGINEERS ΣΥΝΔΕΣΜΟΣ ΓΕΩΛΟΓΩΝ ΚΑΙ ΜΕΤΑΛΛΕΙΟΛΟΓΩΝ ΚΥΠΡΟΥ CO-ORGANISED BY/ ΣΥΝΔΙΟΡΓΑΝΩΣΗ: AGRICULTURAL UNIVERSITY OF ATHENS ΓΕΩΠΟΝΙΚΟ ΠΑΝΕΠΙΣΤΗΜΙΟ ΑΘΗΝΩΝ NATIONAL TECHNICAL UNIVERS ITY OF ATHENS SCHOOL OF MINING &METALLURGICAL ENGINEERING ΕΘΝΙΚΟ ΜΕΤΣΟΒΙΟ ΠΟΛΥΤΕΧΝΕΙΟ ΣΧΟΛΗ MHXANΙΚΩΝ ΜΕΤΑΛΛΕΙΩΝ-ΜΕΤΑΛΛΟΥΡΓΩΝ EDITORS: G.STAMATIS, K.VOUDOURIS, E.VASILEIOU, E.PSOMIADIS, M.DIAKAKIS, M. POTAMIANOU, I.GRIGORIOU, L.ATHANASIADOU ΕΠΙΜΕΛΕΙΑ: Γ.ΣΤΑΜΑΤΗΣ, Κ.ΒΟΥΔΟΥΡΗΣ, Ε.ΒΑΣΙΛΕΙΟΥ, Ε.ΨΩΜΙΑΔΗΣ, Μ.ΔΙΑΚΑΚΗΣ, Μ.ΠΟΤΑΜΙΑΝΟΥ, Ι.ΓΡΗΓΟΡΙΟΥ, Λ.ΑΘΑΝΑΣΙΑΔΟΥ ATHENS 2017/ ΑΘΗΝΑ 2017 INTERACTION BETWEEN GROUNDWATER AND EARTHQUAKES: SEISMIC SITE CHARACTERIZATION AND COASTAL LIQUEFACTION POTENTIAL IN CEPHALONIA (IONIAN SEA) FROM AN UPDATED DATASET 1MICHALAKI K.*, 1KASSARAS I., 2PAPATHANASSIOU G., 3KOUMANTAKIS I., 3VASILEIOU E. and 3MARKANTONIS K. 1 National & Kapodistrian University of Athens, Laboratory of Seismology, [email protected], [email protected] 2 Aristotle University of Thessaloniki, Department of Geology, [email protected] 3 National Technical University of Athens, School of Mining and Metallurgical Engineering, Department of Geological Studies, [email protected], [email protected], [email protected] Ke y words : earthquake, groundwater, liquefaction, PGA, Cephalonia Abstract The subject of this work is the investigation of the interaction between groundwater and seismic waves, resulting in liquefaction of the soil, a particularly dangerous phenomenon. Therefore, estimates of liquefaction potential can significantly contribute to the prevention of such effects and consequently to reduction of the seismic risk. The study area is Cephalonia Island, the most earthquake prone region of Europe. A dataset consisting of seismic ambient noise, accelerograms and datasheet from geotechnical boreholes, obtained after the 2014 earthquake crisis, has been analysed using a series of methodologies. Ambient noise analysis provided amplification functions, Vs30 models and synthetic time histories for numerous sites across the 2014 epicentral area. These were used for the seismic site characterization across the western part of the island and the estimation of the liquefaction potential in the coastal areas of Argostoli and Lixouri, where liquefaction phenomena were observed after the occurrence of the two strongest earthquakes in 2014. The results of the analyses are found to be compliant with the overall arrangement of the 2014 secondary earthquake effects, implying for strong site effects and interaction with the groundwater. Introduction Soil liquefaction, the result of groundwater and earthquakes interaction, is one of the most hazardous secondary seismic effects. Delimitation of the areas and the grade that these are prone to liquefaction are critical issues contributing to the reduction of seismic risk by predicting the possible due damage. The subject of this work is the investigation of the potential of a liquefaction phenomenon to occur in the island of Cephalonia by the interaction between groundwater and ground excitations during a strong earthquake. Cephalonia is located in the Ionian Sea, at the northern edge of the Hellenic Arc, a region with high seismicity rates and complicated tectonics, dominated by the prominent Cephalonia Transform Fault Zone (CTFZ). The area of Cephalonia is considered to be the most seismically prone area of Europe and the Greek territory; according to the Hellenic national seismic code (EAK 2000, 2003) the region is classified to the highest seismic hazard zone (III), for which earthquake acceleration PGA = 0.36g is predicted. On 26/01/2014 and 03/02/2014 two strong earthquakes with magnitude Μw6.1 and Μw6.0 hit the western part of Cephalonia (Figure 1), developing excessively high ground accelerations (e.g. Theodoulidis et al., 2015). Despite the surprisingly high PGA (Peak Ground Acceleration) recorded by local accelerometric instruments, that largely exceeded the seismic code provisions (Kassaras et al., 2017), structural damage was low thanks to the structural characteristics of the constructions of the island (Karantoni et al., 2017). 11th International Hydrogeological Congress of Greece/ Athens 2017 363 The 2014 Cephalonia main-shocks induced severe environmental effects, the main types of which were liquefaction and slope failures, widespread at the western and central part of the island (Papathanassiou et al., 2016), whereas similar phenomena at coastal areas in Cephalonia were triggered by historical earthquakes (Ambraseys, 2009). These effects, and in particular the documented liquefaction at several coastal regions caused a prominent part of the registered damage on the island due to the 2014 earthquakes. The aforementioned motivated our study towards the investigation of liquefaction potential at certain coastal sites in Cephalonia, employing a new dataset obtained in the aftermath of the 2014 earthquake series comprising geotechnical data, earthquake waveforms and free-field ambient noise measurements. Figure 14. Map containing focal mechanisms. Black and white quadrants represent compression and dilatation, respectively. The size of the beachballs is proportional to the earthquakes magnitude. Red stars represent the two major earthquakes of 2014 (26/01/2014, Μw6.1 and 03/02/2014, Μw6.0). Geotechnical Setting Cephalonia is characterized by complex geological structure and intense morphology. The carbonate formations of the Alpine basement, which occupy a large of the island, appear largely karstified, hence leading to a limited surface hydrographic network, the main units of which are in free hydraulic communication with the sea over several kilometres. The geological structure of the island is consisted of limestones, Neogene deposits and molassic sediments, as well as Quaternary deposits (coherent and loose). Moreover, the subsoil formations are divided into three geotechnical units according to their physical and mechanical characteristics (Lekkas et al., 2001; KEDE, 2016): (a) Reclaim ground, (b) Sand and gravel and (c) Clay and marl. 11th International Hydrogeological Congress of Greece/ Athens 2017 364 Figure 15. Geological map of Cephalonia (IGME, 1985). Method and Data Methods Figure 16. Flow chart of the methodology used in this study. The methods applied in this work and their description are outlined below: • The HVSR method: Site response was assessed by applying the HVSR, ‘so-called’ Nakamura’s technique (Nakamura, 1989). The method is based on the Horizontal-to- Vertical Spectral Ratio of earthquake or ambient noise recordings (HVSR) at a site, using a three component seismograph. The method has been proven to reliably approximate soil response and because of its low cost, its easy and quick implementation, it has been 11th International Hydrogeological Congress of Greece/ Athens 2017 365 popular, widely used in substitution of costly and time consuming geophysical surveys and geotechnical boreholes. We applied the Geopsy tools (SESAME, 2004; 2005) to calculate HVSR curves. • The stochastic ground-motion simulation method: The method has been proposed by Beresnev and Atkinson (1997; 1998), is broadly used to determine ground motions in areas where strong motion data are insufficient. The EXSIM algorithm (Boore, 2003) that takes into consideration the geometry and mechanics of the fault was applied. • Inversion of HVSR curves: The method is based on a Monte Carlo approach for inverting HVSR curves towards retrieving 1D viscoelastic models. It was implemented by the use of the ModelHVSR algorithm (Herak, 2008). • Soil liquefaction potential: In order to estimate the soil liquefaction potential we applied the method based on the Factor of Safety (FS) (e.g. Seed et al., 1985; Youd et al., 2001). For the calculation of the FS we applied the method of Idriss and Boulanger (2008), which compares the correlation of the CSR (Cyclic Stress Ratio) caused by the earthquake with the CRR (Cyclic Resistance Ratio), a measure of a soil's liquefaction resistance. The Liquefaction Potential Index (LPI), proposed by (Iwasaki et al., 1978) for the representation of the liquefaction potential within a vertical multi-layer soil column, was adopted. The advantage of LPI is that it quantifies the liquefaction potential, providing a unique value for the whole soil column, instead of many factors of safety (one for each layer) and consequently calculates the spatial probability of liquefaction (Papathanassiou, 2008). To assess the liquefaction potential we used the LiqIT software (GeoLogismiki, 2006), which enables LPI estimation from the combination of observations (i.e. SPT, CPT and Vs). Data Seismic ambient noise data were recorded between 13th May and 16th May 2014, in 80 locations of Cephalonia, in Paliki and the western part of Argostoli peninsulas during a field work conducted by the Seismological Laboratory of the National and Kapodistrian University of Athens (Kassaras et al., 2017). For the approximation of the geotechnical conditions prevailing on the island we used a large number of geotechnical borehole data, provided by the